These missions are getting tougher and unfortunately, I was only able to get 2 stars on these. In most cases, I had beaten all the defense but simply ran out of time. Hopefully, Disney will increase the time allotted for level 7+ battles, similar to what they did in Missions 24. There are just too many buildings to destroy in under 2 minutes. The same issue exist in PvP battles also.

Here's the "How to 3 Star" from the Empire side. Empire seems initially stronger but I found it easier to win with Rebels during the later missions. Darth Vader is also much stronger than Luke Skywalker as can be observed in missions 20.